1. Field of the Invention
The invention relates to a mixer circuit. More particularly, the invention relates to a mixer for a radio frequency receiver of a cellular phone.
2. Description of the Related Art
A central component of a receiver for a radio frequency signal (RF signal) is a mixer which translates a signal at a first frequency to a different, second frequency. For instance, the receiver receives the RF signal having a high radio frequency, and the mixer translates the RF signal, by mixing it with a local oscillator (LO) signal, to a signal (IF signal) having an intermediate frequency (IF). The intermediate frequency is generally lower than the radio frequency. The frequencies within the receiver are related by, fIF=fRF−fLO or fIF=fLO−fRF.
The mixer, which is a nonlinear device, in fact generates an output signal that includes more frequencies than the frequencies (fRF−fLO) of the RF signal and the local oscillator signal. The output signal is usually filtered to block undesired frequencies which include the original frequencies, their harmonics and their sum and difference frequencies.
The mixer used in such a receiver should ideally exhibit several desired parameters and characteristics which describe the performance of the mixer. For example, a mixer should exhibit a high power gain, a low noise figure and the capability of handling large input signals without intermodulation distortion (IMD). The power gain is defined as the ratio of the signal power at the output port to the signal power at the input port. The noise figure is defined as the ratio of the signal-to-noise ratio (SNR) at the input port to the signal-to-noise ratio at the output port.
The intermodulation distortion refers to undesired frequency components which are caused when a signal having two or more sinusoidal frequencies f1, f2 is applied to a nonlinear RF circuit. The output signal of such a circuit contains the additional, undesired frequency components which are called intermodulation products. The output signal of the circuit will contain, for example, frequency components at DC, f1, f2, 2f1, 2f2, 3f1, 3f2, f1+/−f2, 2f1+/−f2 and 2f2+/−f1. The frequencies 2f1 and 2f2 are the second harmonics, the frequencies 3f1 and 3f2 are the third harmonics, the frequencies f1+/−f2 are the second-order intermodulation products, and the frequencies 2f1+/−f2 and 2f2+/−f1 are the third-order intermodulation products. The third-order intermodulation products are close to the fundamental frequencies f1 and f2 and fall within a bandwidth in which the circuit amplifies, producing distortion in the output signal. A mixer exhibits the same nonlinear performance in addition to the frequency translation.
A parameter to evaluate the third-order intermodulation products is the third-order intercept point (IP3). This point is defined by means of a graphical analysis using the output power of the third-order intermodulation product as a function of the input power and the output power of the fundamental component at f1 as a function of the input power. The intercept point is defined as the point at which the two (linearized) graphs intercept. The higher the intercept point, the better the suppression of the third-order intermodulation product and the less disturbed is the output signal of the amplifier. The mixer should exhibit, among others, a high conversion gain, a low noise figure and also a high third-order intercept point (IP3). The conversion gain is defined as the ratio of the output power of the IF signal to the input power of the RF signal.
The mixer is configurable to operate in receivers which are adapted for various applications. For example, the mixer can be used in TV receivers or in phones for a radio communications system. One example of a radio communications systems is a cellular system that is in accordance with a particular standard, such as “Global System for Mobile Communications” (GSM), “Advanced Mobile Phone System” (AMPS) or “Code Division Multiple Access” (CDMA).
These standards have different requirements and specifications for the mixer with respect to linearity, noise figure and intermodulation distortions. Particularly the CDMA standard requires that the mixers in CDMA phones have simultaneously both a high IP3 and a low noise figure.